US cop kills retiree in 'tragic accident'

A Florida police officer has shot and killed a 73-year-old retired librarian during an exercise with local residents that was meant to involve simulated lethal force, authorities say.

Mary Knowlton was hit by a live round fired by an officer on Tuesday night during a role-playing exercise at an event hosted by the Punta Gorda Police Department for about 35 members of the community, the agency said in a statement on Wednesday.

The married mother of two sons was a retiree from Minnesota and had signed up for the city's Citizens Academy as a show of support for law enforcement after deadly attacks on police in Dallas and Baton Rouge, Louisiana, son Steve Knowlton told NBC News.

The academy consists of a series of sessions designed to provide insight into city government and includes site visits to various agencies, according to Punta Gorda's website.

Mary Knowlton's neighbourhood had recent break-ins, and she was playing the role of an intruder when she was fatally shot, said her son.

Police Chief Tom Lewis gave reporters few details about the revolver fired in the incident. It had been used in similar exercises before, and officials thought only blank rounds were available for it, Lewis said.

"We were unaware that any live ammunition was available to that officer at the time," the chief said.

The officer, who has not been named, is on administrative leave while the Florida Department of Law Enforcement investigates.

"He's very grief-stricken," said Lewis, who added that counselling will be made available next week to others affected by the incident.

Punta Gorda is a city of about 18,000 people located on Florida's west coast, about 50km north of Fort Myers.

The chief described Mary Knowlton as "a phenomenal person in this community" who attended many local events.

"Everyone involved in this accident is in a state of overwhelming shock and grief," Lewis said in an earlier statement.